Graphite materials are chemically stable and are excellent in electric and thermal conductivity and mechanical strength at high temperature, and therefore, are widely used for electrodes for steel making, electrodes for arc melting and reducing of high purity silica and electrodes for aluminum refining. Graphite has a crystal structure formed by stacking of carbon hexagonal planes generated by growth of carbon hexagonal rings by sp2 hybridized orbital of carbon atoms, and is classified into a hexagonal system and rhombohederal system depending on the form of lamination. The both systems show good electric and thermal conductivity since a carrier concentration and carrier mobility of free electron and holes in the carbon hexagonal planes are high.
On the other hand, since the carbon hexagonal planes are weakly bonded to each other by so-called Van der Waals force, slip occurs relatively easily between the planes, and as a result, graphite has lower strength and hardness as compared with those of metallic materials and has self-lubricating property.
Since natural graphite produced naturally is a polycrystalline material, breakdown occurs at an interface of crystal grains and natural graphite is produced in a flaky form, not in a massive form having sufficient hardness and strength. Therefore, generally natural graphite is classified by its particle size and is used as an aggregate (a filler).
On the other hand, in order to use graphite in various applications mentioned above by making use of excellent characteristics thereof, it is necessary to produce a graphite structure having practicable strength and hardness. Since it is difficult to obtain such a structure from natural graphite alone, various so-called artificial graphite materials have been developed and put into practical use.
(General Method for Producing Artificial Graphite Materials)
Artificial graphite materials are produced by mixing a filler as an aggregate and a binder and subjecting the mixture to molding, baking for carbonization and graphitization treatment. It is essential that both of the filler and the binder remain as carbon after the baking for carbonization so as to give high carbonization yield, and a suitable filler and binder are selected depending on applications.
A pre-baked petroleum coke, a pre-baked pitch coke, a natural graphite, a pre-baked anthracite, a carbon black and the like are used as a filler. These fillers are kneaded with coal tar pitch, coal tar, a polymer resin material, or the like and molded into a desired form by extruding, casting, pressing or the like method.
A molded material is baked for carbonization at a temperature of 1000° C. or more in an inert atmosphere and then baked at a high temperature of 2500° C. or more for developing a graphite crystal structure and graphitizing. During the baking for carbonization, the starting material are subject to decomposition, and moisture, carbon dioxide, hydrogen, and hydrocarbon gases are generated from component elements other than carbon such as hydrogen and nitrogen, and therefore, the baking is controlled to be a low temperature elevating rate, and generally a very long period of time of 10 to 20 days for heating up and 5 to 10 days for cooling, totally 15 to 30 days is necessary for production.
Graphitization process is carried out by electric heating with a large-sized oven such as an Acheson electrical resistance oven. Also in the graphitization process, a period of time of 2 to 7 days for electric heating and 14 days for cooling, totally 16 to 21 days is necessary. Totally about two months is required for production including preparation of a staring material, molding, baking for carbonization and graphitization. (Non-patent Document 1)
In general artificial graphite, a filler added in a molding step is easily formed evenly in a certain direction and crystallinity is enhanced as carbonization and graphitization proceed. Therefore, anisotropy tends to be increased and as a result, a bulk density and a mechanical strength tend to be decreased.
Both of the filler and binder to be used are hydrocarbon substances to be carbonized after heat treatment and are roughly classified into easily graphitizable materials to be easily graphitized due to a chemical structure thereof and hardly graphitizable materials hardly graphitized due to crosslinking of a benzene ring in a structure thereof.
(Method for Producing High Density Isotropic Graphite Material)
Examples of means for achieving high density are to use a filler capable of being easily graphitized such as mesocarbon microbeads comprising extracted matter of mesophase, gilsonite coke or carbon beads, and then to adjust particle size distribution thereof, to enhance compatibility thereof with a binder pitch, or to repeat impregnation treatment thereof. Also, in order to impart isotropic property, application of isotropic pressure with cold isostatic pressing equipment at the molding stage is effective and is a general method. In order to further increase a density, a process for impregnating the material with a binder pitch again after the graphitization and repeating the graphitization treatment has been carried out, but in this process, a total period of time required for production is as extremely long as 2 to 3 months.
In the case of use for electrode materials and nuclear power application, purity of a graphite material is critical, and it is necessary to carry out a treatment for securing high purity with halogen gas such as chlorine gas at a temperature of as high as around 2000° C. By the treatment for securing high purity, a concentration of impurities is decreased from about several hundreds ppm to about several ppm.
A starting material to be used for producing general artificial graphite and high density isotropic graphite is in a liquid or solid form. In molding, carbonizing and graphitizing processes, a liquid phase-solid phase reaction or a solid phase reaction proceeds predominantly. These hydrocarbon based materials expand its benzene ring network due to dissipation of elements such as hydrogen, oxygen and nitrogen therefrom, and approximates a graphite crystal structure by growth and stacking of carbon hexagonal planes. Particularly in the graphitization process, which is a solid phase reaction, an extremely long reaction time at a temperature of as high as 2500° C. or more is required.
In the case of artificial graphite and high density isotropic graphite, the graphitization proceeds in a liquid phase or a solid phase, and therefore even if heat treatment is carried out for a long period of time at a temperature of as high as 3000° C. or more, complete crystallization (graphitization) is difficult, a density of the graphite does not reach a theoretical density of 2.54 g/cm3, and there is a limit in a crystal size thereof.
(Heat Treatment of Polymer Resin Material)
In the case of a carbon fiber produced using a resin such as polyacrylonitrile (PAN), coal or petroleum pitch as a starting material, such starting material of a polymer material are draw into a fiber and then carbonized and graphitized in the following heat treatment. In addition, a highly oriented graphite film having high crystallinity can be produced by depositing or applying boron, rare earth element or a compound thereof to a polyimide film or a carbonized polyimide film, laminating a plurality of films and then carrying out baking while applying pressure to the film surface in the vertical direction thereof at a temperature of 2000° C. or more in an inert atmosphere. However, an upper limit of the film thickness is several millimeters. (Patent Document 3)
(Method for Precipitating Highly Oriented Graphite in Glassy Carbon)
In JP 2633638 B (Patent Document 6), it is disclosed that a graphite in the form of like bean jam of Monaka of a Japanese-style confection is precipitated in a glassy carbon by means of molding a thermosetting resin into a thick plate by hot press or the like, forming the resin into a glassy carbon by carbonization treatment and subsequently subjecting the glassy carbon to hot iso static pressing treatment. In this method, it is necessary to control thickness of the glassy carbon to about 6 mm in order to enable baking and also necessary to break a shell of the glassy carbon after generation of graphite in order to take out a graphite precipitate.
(Method for Producing Graphite Material by Vapor Phase Growth)
There is a method for producing carbon and a graphite material through vapor phase growth by using hydrocarbon and hydrogen gas as starting materials and a reactor such as CVD (Chemical Vapor Deposition) equipment and bringing the starting materials into contact with a metal catalyst at high temperature. Examples of carbon materials to be produced by vapor phase growth are a vapor-phase-grown carbon fiber, a carbon nanotube, a carbon nanohorn, fullerene and the like.
In the case of a vapor-phase-grown carbon fiber, by suspending an oxide of transition metal having a size of several hundreds angstrom in a solvent such as an alcohol and spraying the solvent onto a substrate and drying it, the substrate carrying a catalyst is produced. This substrate is put in a reactor and a hydrocarbon gas is flowed thereinto at a temperature of 1000° C., thus growing a carbon fiber from the surface of the transition metal on the substrate by vapor phase reaction. Alternatively there is a case of letting a mixture of a gas of organic transition metal compound and a hydrocarbon gas flow into a reactor of about 1000° C. (Patent Document 1)
A graphitized fiber is obtained by subsequently heat-treating the carbon fiber obtained by vapor phase growth at high temperature of 2000° C. or more in an oven for graphitization treatment. (Patent Document 2) In order to produce a graphitized fiber directly by vapor phase growth, a reaction temperature of around 2000° C. is required. However, in such a temperature range, a transition metal as a catalyst is liquefied and vaporized, and a function of the catalyst is not exhibited. Therefore, generally graphitization is carried out separately after carbonization at low temperature.
(Carbon Nanotube)
A carbon nanotube is a very minute substance having an outer diameter of the order of nanometer and comprising cylindrical shape carbon hexagonal plane having a thickness of several atomic layers, which was found in 1991. (Non-patent Document 1) It is known that this carbon nanotube exists in a deposit generated on a negative electrode due to arc discharge of a carbon material such as a graphite, and this carbon nanotube is produced by using a carbon material such as a graphite as a positive electrode and a heat resistant conductive material as a negative electrode and carrying out arc discharge while adjusting a gap between the positive electrode and the negative electrode in response to growth of a deposit on a negative electrode. (Patent Document 4)
A carbon nanotube is generated by arc discharge. However, a large-sized reactor is required and yield obtained is extremely low, and therefore, a mass production method has been studied. Generally in arc discharge of carbon to be used for production of a nanotube, plasma in a state where carbon molecular species such as C, C2 and C3 being contained is generated in a reactor fully filled with an inert gas, and, in the next stage, these carbon molecular species are solidified into soot, fullerene, a nanotube or a high density solid. Therefore, yield of nanotube is increased by optimizing a partial pressure of gases in a chamber and a plasma temperature. (Patent Document 5)
A tube composed of carbon hexagonal planes (graphene sheet) is CNT, and a carbon nanotube comprising a single layer graphene sheet is called a mono-layer CNT or SWCNT (Single-walled Carbon Nanotube) having an outer diameter of about 0.5 nm to about 10 nm, and a carbon nanotube comprising multi-layer graphene sheets is called a multi-layer CNT or MWCNT (Multi-walled Carbon Nanotube) having an outer diameter of 10 nm to 100 nm. Thus carbon nanotubes are classified in such a manner. Currently most of commercially available carbon nanotubes are multi-layer CNT, which are a mixture with carbon fibers and graphite fibers that do not form a tube.
Methods for producing a carbon nanotube are explained systematically as follows.
1) Arc Discharging Method
High voltage is applied between carbon electrodes in vacuo or under reduced pressure to cause arc discharging and deposit carbon vaporized at locally super high temperature (4050° C.) on the negative electrode.
2) Laser Vaporization Method
Laser is emitted to a mixture of carbon and a catalyst in vacuo or under reduced pressure to vaporize carbon at a locally super high temperature (4050° C.), and grow the vaporized carbon into CNT on the catalyst.
3) Chemical Vapor Phase Growth Method
CNT is precipitated on a catalyst by passing a carbon-containing gas (hydrocarbon) and a metal catalyst through a reaction tube heated to 1000-2000° C.
4) Other Methods Such as SiC Surface Decomposition Method and Polymer Blend Spinning Method
Fullerene is a spherical molecule comprising 60 carbon atoms, and one having a structure similar to a soccer ball is called C60, one having more than 60 carbon atoms in a cage is called a high-order fullerene, and one containing metal in a cage is called a metal-incorporated fullerene. Fullerene is extracted from a vaporized carbon obtained by applying a high voltage between carbon electrodes in vacuo or under reduced pressure to cause arc discharging and vaporizing at locally super high temperature (4050° C.) by the arc discharging method in the same manner as in CNT. In addition, at an initial stage of the arc discharging, fullerene is generated by combusting a gas mixture of a carbon-containing gas (hydrocarbon), oxygen and argon under reduced pressure by a combustion method.
Also, nanocarbon materials such as graphene composed of one carbon hexagonal plane and a carbon nanohorn obtained by forming graphene into a tube of a circular cone shape are reported. However, any of them are produced by the same method as in fullerene, and in many cases, carbon materials other than CNT are produced secondarily and a selective production method has not been established.